The present invention relates generally to the field of process control instruments. More particularly, the present invention relates to systems and methods for facilitating serial communication in a transmitter.
Transmitters in the process control industry typically communicate with a controller over a two-wire circuit or control loop. The transmitter receives commands from a controller over the two-wire control loop and sends output signals representative of a sensed physical parameter back to the controller. The transmitter itself can include a number of devices such as master nodes or microprocessors, and slave nodes or peripheral devices such as sensors, valve positioners and communications circuitry. Communication between the various devices of the transmitter typically takes place on a data bus which includes a clock line and a number of data lines. Because of concerns about current consumption and complexity, it is important to minimize the number of data bus lines required for communication between the various devices.
In many process control instruments, the sensor of the process control instrument must be isolated from measurement circuitry or other devices of the process control instrument for safety purposes. An isolation barrier is used to electrically isolate the sensor. The sensor receives power and communication through the barrier and the barrier prevents large electrical discharges from impairing the measurement circuitry. Typically, the barrier only allows communication on a single data line. While necessary for safety or other purposes, inclusion of the isolation barrier reduces the number of electrical connections between the sensor and the other devices, and thus increases the complexity of communications between the sensor and the other devices.
The present invention includes first and second devices coupled to a data line. The first and second devices communicate serially by controlling a duty cycle of the data signal on the data line during cycles of the data signal. Generally, one device on the data generates a clock signal and any device on the line can modulate the clock signal to transmit data. In some embodiments, the first device is a master node (or microprocessor) and the second device is a slave node (or peripheral device). Frequently, both the first and second devices comprise integrated chips which communicate over the data line. In other embodiments, the data line is a clock line which carries a clock signal. In these embodiments, the first and second devices communicate serially by controlling the duty cycle of the clock signal during cycles of the clock signal. In some embodiments, the master node initiates the clock cycle so that the clock signal is supplied at a substantially constant frequency.